This invention generally relates to a method of reactive ion etching for the manufacture of semiconductor integrated circuits, and more particularly to a method of reactive ion etching which is useful for fine processing of molybdenum (Mo) and molybdenum silicide, which may be used for the gate material and the wiring material of integrated circuits.
As the density of integration is increased, the electrical resistance of silicon gates and wiring become significant because their cross-sectional areas are greatly decreased. From another point of view, gate and wiring material is required to withstand high temperatures (about 1273 K.) for presently-used processes of integrated circuit manufacture. In recent years, in order to overcome such problems, molybdenum (Mo) and its silicon compound molybdenum silicide (MoSi.sub.2), which are high-melting-point materials, have been used in place of Si. For processing these materials, reactive ion etching is employed, because it is faithful to the dimensions of the photoresist mask. At first, a mixture of CF.sub.4 and O.sub.2 gases was used in the reactive ion etching of molybdenum silicide. However, this mixture is not suitable for fine processing of this material because of the difficulty of avoiding undercutting (see FIG. 4B for an example of undercutting).
In recent years, reactive ion etching using a mixture of CCl.sub.4 and O.sub.2 gases has been reported, see K. Nishioka, et al., "Anisotropic Etching of MoSi.sub.2 /Poly-Si Double Layer Films with Chlorinated Gas Plasma," Proceedings of Symposium on Dry Process 1982, at 51. According to this report, since the reaction products are chlorides of molybdenum or silicon or compounds of molybdenum with chlorine and oxygen, if only CCl.sub.4 is used, non-volatile carbon will accumulate on the surface of the molybdenum silicide and decrease the etch rate. Consequently, O.sub.2 is added to CCl.sub.4 in order to promote the etching by changing carbon into CO and CO.sub.2, which are volatile. However, the range of permissible oxygen addition is limited, because if excess oxygen is added, this causes oxidation of the photoresist mask. Therefore, in the conventional method using CCl.sub.4 and O.sub.2, it is necessary to keep the O.sub.2 flow rate below a certain level. As a result of this limitation, a certain degree of carbon accumulation on the surface of the molybdenum silicide is almost inevitable, and ion bombardment is required in order to remove the accumulated carbon.
Since the strength of ion bombardment varies inversely with the pressure of the gases, etching has to be carried out at a comparatively low pressure. But, the etch rates of photoresist and SiO.sub.2 are mainly determined by the degree of ion bombardment. Accordingly, if etching is carried out at low gas pressure (a high level of ion bombardment), the etching selectivity ratio of molybdenum silicide over SiO.sub.2 or photoresist decreases undesirably (because the etch rates of SiO.sub.2 and photoresist increase). As the density of circuit integration increases, it has been necessary to make the gate oxide layer of SiO.sub.2 proportionately thinner. So, when molybdenum silicide is used as the gate, this reduced etching selectivity ratio can be unsatisfactory.